This project will use the techniques of somatic genetics to analyze the events leading to the expression by the neoplastic cell of a new surface antigen. The GIX antigen on murine thymomas will be studied as a model since it is the best characterized system serologically and genetically. Immunoselection will be used to isolate cell lines which have defects in GIX expression. The techniques of somatic cell genetics will be used to identify the genes responsible for the variant phenotype and to determine their number, dominance, and relation to known genes governing GIX expression. The variants will then be used in experiments designed to determine which host genes are necessary for expression of GIX upon overt Gross virus infection.